Animal confinement systems that provide a stimulus to an animal approaching a predetermined boundary to encourage the animal to remain within the predetermined area defined by the boundary are well known and are commercially available. Generally, there are two such types of systems being sold or otherwise available today--a wire system and a wireless system. The first of these relies on the placement of a transmitter wire above or under ground to define the boundary of a containment area. The transmitter wire transmits an electrical current that in turn sets up an electrical field about the wire. An animal, such as a dog, is equipped with a receiver that senses the electrical field generated by the current in the transmitter wire. The receiver, based upon the strength of the electrical field signal received (and, hence, the distance from the transmitter wire) can be programmed to provide an auditory and/or electrical stimulus. In other words, the receivers worn by the dog typically can be programmed to emit "beeps" at a certain distance from the transmitter wire and/or it can be programmed to provide an electrical shock to the dog when the dog gets within a predetermined distance of the transmitter wire. Usually, the receiver is placed on a collar worn by the dog.
Another known system relies on a single stationary transmitter to broadcast a signal. The transmission is substantially omnidirectional with a substantially uniform signal strength in all directions. The broadcast signal is received by a receiver worn by the dog. Again, the receiver measures the signal strength, determines the distance of the receiver and hence the dog from the transmitter, and applies the appropriate stimulus to the animal if necessary. Conversely, there could exist a centrally located receiver that would receive a signal broadcast by a transmitter worn by the animal. The centrally located transmitter would then measure the signal strength of the broadcast signal and determine whether the animal was approaching the boundary and whether a stimulus should be applied according to pre-determined criteria. This latter variant of such a confinement system would be more expensive than the first variant because it would require a transmitter and a receiver to be placed both on the animal and at the central location, unlike the first, which requires a single transmitter and a single receiver.
A third confinement system that has been proposed relies on ultrasound signal transmissions. With the ultrasound system, a responder worn by the animal is responsive to an ultrasonic signal broadcast by a centrally located transmitter. The distance of the animal from the transmitter is measured by the round trip time of the transmitter signal from the transmitter to the responder and back to the transmitter. When the round trip time indicates that the animal is approaching or within the predetermined stimulus application area, the centrally located ultrasound transmitter will broadcast a signal to the responder directing that a stimulus be applied to the animal. As with the wireless system just described, the ultrasound system also is constrained to use a substantially circular confinement area.
Another wireless system has been proposed that relies on the broadcast and reception of electromagnetic, that is, radio, signals to define the boundary and the stimulus application area. A multiple receiver/multiple antenna unit is worn by the animal. Through a complex vector analysis of the signals received by the unit the exact distance from the animal to the transmitting antenna can be determined and the appropriate stimulus can then be applied. This particular wireless system consumes large amounts of power, necessitating frequent battery recharging and/or replacement in the unit worn by the animal.
Yet another wireless system is known to the prior art. In this system, a transmitter modulates the electromagnetic signals produced by the transmitter to produce non-random patterns of signals. A comparator in the receiver compares the signal levels of the received signals with a predetermined signal level held in memory and produces an output indicative of the signal degradation. The receiver also compares the phase of the modulation of the received signals with a reference phase held in memory. Through additional processing of the output signals the distance of the animal from the transmitter is determined and the appropriate stimulus is applied.
Typically, with either of the afore described systems, some training period is necessary before the animal will learn the boundary beyond which it is not to pass. Often this involves marking the boundary with flags such that the animal will have a visual representation of the boundary to associate with the stimulus received from the collar.
These commercially known systems, while being admirable attempts to devise a workable confinement system for pets, suffer, however, from several notable drawbacks. In the first or wire system, the requirement of the transmitter wire means that the boundary, once placed, is relatively fixed and the system itself is likewise also fixed. In other words, once the wire has been used to define the boundaries, redefining the boundaries requires that the wire be moved, often by digging it up and reburying it. The system suffers, then, from being unable to be moved easily, if at all, so as to define temporary boundaries, such as when a pet is taken on a family vacation.
In addition, when the wire system is used, the homeowner's entire yard effectively cannot be used by the pet. For example, placement of the transmitter wire underground will usually be accomplished entirely on the pet owner's own property, barring excessively good neighborly relations. Since it is typically desired to keep the pet within one's own yard, the stimulus will be applied within several feet of approaching the transmitter wire. In essence, then, an area defined by the yard's perimeter by several feet wide will be unaccessible to the pet. In small yards in particular, this can mean that a substantial percentage of the yard will be inaccessible to the animal for play and exercise.
An additional drawback is that the wire system can be rather expensive when the cost of obtaining the necessary wire to define the area as well as the installation costs are calculated.
Finally, if the confined animal chances to escape the containment area, the animal will receive a warning stimulus if and when it tries to return to the pre-defined area since the stimulus is given when the animal is within a certain distance of the transmitter wire, regardless of which side of the wire it is standing. In other words, as the animal tries to return home, it will receive a stimulus from the receiver as it approaches the transmitter wire and enters the defined distance at which a stimulus will be given. The animal will then be discouraged from coming home if it should happen to escape, thus defeating the purpose of trying to keep the animal within the home yard in the first place.
The wireless systems discussed above, while potentially portable, do not allow for any flexibility in the boundary, which is substantially defined by the directional properties of the transmitter and is most often circular in configuration about the transmitter location. Thus, while a wireless system can be used readily on a vacation to define a pet area, it suffers from the lack of an ability to take advantage of a homeowner's entire yard; or if the entire yard is within the circular containment area, usually a portion of a neighbor's yard is also since rarely, if ever, are yards circularly configured. In addition, particularly with dogs, it is often desired to keep the pet away from a newly planted area, whether lawn or other forms of landscaping. If the newly planted area happens to be within the defined circular boundary, only by redefining the circular boundary to lie closer to the transmitter can the dog be kept away from the newly planted area. Such a boundary redefinition, however, would typically remove a great deal of otherwise useable lawn area from the confinement area, which is rarely an acceptable solution. In other words, the presently available systems do not provide a method of setting off preselected areas where the pet is not allowed within the larger allowable areas where the pet can enter.
Prior art confinement systems can be found in U.S. Pat. Nos. 3,753,421 to Peck; 3,980,051 to Fury; 4,898,120 to Brose; 5,067,441 to Weinstein; and 5,381,129 to Boardman, among others. Reference can be made to those patents and others for a more complete description and understanding of the prior art.
Thus, while the prior art contains many inherent deficiencies, a presently available technology offers the opportunity to develop a more useful system. For example, development of Satellite Positioning Systems (SATPSs), such as the Global Positioning System (GPS) in the United States and the Global Orbiting Navigational System (GLOSNASS) in the former Soviet Union, has allowed location coordinates of an object on or near the Earth to be determined with improved accuracy. Under normal circumstances, these systems allow coordinates to be determined with an accuracy of no more than 30 meters.
A Satellite Positioning System (SATPS) is system of satellite signal transmitters, with receivers located on or near to the earth's surface, that provides means for determining the location of a receiver and/or time of observation. Two operational systems currently in place are the Global Positioning System and the Global Orbiting Navigational System.
The Global Positioning System (GPS) is a satellite-based navigational system developed by the United States Department of Defense. The GPS system employs 24 satellites, 4 of which share one of six orbits around the earth at a radius of 26,560 kilometers and are approximately circular. The system allows that three or more satellites are visible on most points of the earth's surface at any given time. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the transmitted signals.
Each GPS satellite transmits two spread spectrum, L-band carrier signals: an L1 signal having a frequency f1=1575.42 MHz. These two frequencies are multiples of a base frequency, f0=1.023 MHz. These two transmitted signals allow for partial compensation of propagation delay of such a signal through the ionosphere as put forth by MacDoran in U.S. Pat. No. 4,463,357. The L1 signal is binary phase shift key (BPSK) which is modulated by two psuedo-random noise (PRN) codes. The two PRN codes are modulated in phase quadrature, and are termed C/A-code and P-code. The L2 signal from each satellite is BPSK modulated by only the P-code.
Use of the PRN codes allows use of a plurality of GPS signals. A signal transmitted by a particular GPS satellite is selected by creating and matching the PRN code for that particular satellite. PRN codes for all satellites are known and stored in each GPS satellite signal receiver. A first P-code for each GPS satellite is a relatively lengthy code which includes an associated clock rate of 10 F0=10.23 MHz. A second PRN code for each GPS satellite, the C/A code, or clear/acquisition code, is short code which allows for rapid satellite signal acquisition and has a clock rate of F0=1.023 MHz. The C/A-code for any GPS satellite has a length of 1023 chips or time increments before this code repeats. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7 days) before this code portion repeats. Accepted methods for generating the C/A code and P-code are set forth in the document GPS Interface Control Document ICD-GPS-200, published by Rockwell International Corporation, Satellite Systems Division, Revision A, Sep. 26, 1984, which is incorporated by reference herein.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite and an almanac for all GPS satellites, with parameters providing corrections for ionospheric signal propagation delays suitable for single frequency receivers and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 Baud. A useful discussion of the GPS techniques for deciphering position information of GPS satellite signals can be found in the NAVSTAR Global Positioning System, Van Nostrand Reinhold, New York, 1992, pp. 1-90.
A second configuration for global positioning is the Global Orbiting Navigation Satellite System (GLONASS), developed by the former Soviet Union. GLONASS also uses 24 satellites, but they are spread evenly over three circular orbits of radii of about 25,510 kilometers. The GLONASS system also uses two carrier signals L1 and L2 with frequencies of f1=(1.602+9 k/16) GHz and f2=(1.246+7 k/16) GHz, where k is (0, 1, 2, 3, . . . 23) is the satellite number or satellite tag. The methods for receiving and analyzing the GLONASS signals are similar to the methods used for the GPS signals.
Reference to a Satellite Positioning System or SATPS herein refers to a Global Positioning System, to a Global Orbiting Navigation System, and to any other satellite-based navigational system which is compatible with the present invention.
A SATPS uses transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. A single receiver of such signals is capable of determining receiver absolute position in an Earth-centered or -fixed coordinate system.
Two or more receivers can be used to more accurately determine the relative positions between the receivers or stations. This method, known as differential positioning, is far more accurate than absolute positioning, provided that the distances between these stations are substantially less than the distances from these stations to the satellites. In differential position determination, many of the errors in the SATPS that compromise the accuracy of absolute position have little or no effect on differential positioning due to a process of partial error cancellation.
An SATPS antenna receives a SATPS signals from a plurality of SATPS satellites and passes these signals to an SATPS signal receiver/processor, which (1) identifies the SATPS satellite tag, or satellite source, for each SATPS signal, (2) determines the time at which each SATPS signal arrived at the antenna, and (3) determines the present location of the antenna based upon this information and from information stored on the ephemerides for each identified satellite.
In order to further improve the accuracy provided by SATPS location determination, differential GPS (DGPS), and more generally differential SATPS (DSATPS) has been introduced and used. A DSATPS can provide locations with inaccuracies as low as a few meters, or lower in some instances. Implementation of a DSATPS requires than an SATPS reference station, whose location coordinates are known with high accuracy (to within a fraction of a meter) be provided to receive the normal SATPS signals from SATPS satellites. The reference station compares its known psuedorange (psuedorange refers to the distance between a SATPS receiver antenna and a given SATPS transmitting satellite), based on its known location and known satellite and clock biases, with the psuedorange computed using the acceptable SATPS signals received from each visible satellite. The difference, called a psuedorange correction, between the known psuedorange and the computed psuedorange is transmitted for each such SATPS satellite, along with an indicium that identifies that satellite. A mobile SATPS station within 100-200 kilometers (km) of the reference station receives and uses these psuedorange corrections to correct its own SATPS-determined psuedorange values for each acceptable satellite signal. The psuedorange corrections must be received and processed at the mobile station.
While psuedorange correction techniques currently employed with DSATPS do provide for positional accuracy of the mobile SATPS receiver, they also require additional logic and circuitry, which in turn leads to increased cost. Additionally, a consumer is forced to pay for access to the DSATPS correction signals from the reference stations currently in use (such a service is available from Omnistar, of Houston, Tex.). There is a need for a low cost method of determining the position of a mobile SATPS receiver relative to a reference SATPS receiver when in close proximity to each other (within 1-20 kilometers).
Although SATPS-assisted determination of location and/or time coordinates is quite well known, only a few patents disclose procedures for using differential positioning techniques in order to improve the accuracy of relative location coordinates of a mobile receiver with respect to a given reference receiver. Rather, psuedorange corrections and such are generally used to improve the accuracy of the global positioning coordinates produced by SATPS. For example, in U.S. Pat. No. 5,495,257 Loomis discloses a method for enhancing the accuracy of global location coordinates computed for a mobile SATPS receiver by using psuedorange corrections generated by a reference SATPS station, whose location coordinates are known with a high degree of accuracy. Similar methods are disclosed by Babu in U.S. Pat. No. 5,451,964, and Kyrtsos in U.S. Pat. No. 5,490,073.
However, one method for determining the relative position of two SATPS receivers is disclosed in U.S. Pat. No. 5,202,829 by Geier. This system solves the problem of finding the relative positions of SATPS receivers by time-tagging candidate psuedoranges, aligning like-tagged psuedoranges from respective GPS receivers, and subtracting the difference between corresponding psuedorange to arrive at accurate relative position determinations. Although this method does provide for accurate relative position determination, there are drawbacks. Namely, complex logic and circuitry is required to use the psuedorange differences to arrive at a relative position. This results in additional cost and computational time. As a matter of fact, Geier's system utilizes a personal computer to perform this function.
Thus, is a need for a relative positioning method provided with simple, low-cost logic. Furthermore, it would be desirable to have a confinement system was portable, that would take full advantage of the available area in a pet owner's yard; that was flexible in operation so as to accommodate desirable boundary changes in a confinement area, such as when new yard plantings occurred; that enabled the operator to designates "islands" of non-allowed areas within a larger allowable use area for the animal; and that consumed reduced amounts of power so as to provide long battery life.